The present invention relates to an induction heating apparatus such as an induction heating cooking unit in which load of high conductivity and low permeability, e.g., an aluminum pot, can be heated efficiently; and a induction heating type water heater, humidifier, an iron or the like.
As for a conventional induction heating apparatus, e.g., an induction heating cooking appliances, a technology capable of preventing both a pot vibration noise and reduction of power factor while heating an aluminum pot is disclosed, e.g., in Japanese Patent Laid-Open Publication No. 1989-246783, and a technology for reducing a switching loss and for heating an aluminum pot with high-frequency wave is disclosed, e.g., in Japanese Patent Laid-Open Publication No. 2001-160484.
FIG. 9 is a circuit included in Japanese Patent Laid-Open Publication No. 1989-246783 supra. In FIG. 9, bridge circuit 2, which rectifies AC(alternate current) power supply voltage of 100V to output DC(direct current) voltage, includes two thyristors 3, 4 and two diodes 5, 6. Thyristors 3, 4 control a conduction angle and, upon initiating the operation, reduce the DC voltage down to about 20V to set a low output power. And if load detector 24 detects an existence of a suitable load, output controller 26 controls the output power by varying the DC voltage.
Furthermore, input waveform shaper 23 drives transistor 10 to make an input current of a predetermined waveform based on signals outputted by input setting unit 25 and input current detector 22, thereby increasing the power factor. The enhancement of the power factor is achieved by accumulating energy in choke coil 8 when transistor 10 is turned on and then by transferring the energy to capacitor 11 via diode 9 when transistor 10 is turned off.
Also, in order to heat an aluminum pot, a frequency of a current passing through heating coil 18 is increased from 20 kHz to 50 kHz by varying the number of turns of heating coil 18 and the capacitance of resonant capacitor 19.
However, the prior art described above has many problems: that is, there is required a costly and complicated circuit structure capable of changing the number of turns of heating coil 18 in order to selectively heat both an aluminum pot and an iron pot; and there incurs a large switching loss in switching devices 15, 17 because the driving frequency thereof is required to be set at same 50 kHz in order to accommodate the resonant frequency of 50 kHz; and if a resonance point tracking method is adopted to decrease the switching loss, additive circuits, such as a control circuit therefor and a power supply voltage varying circuit for output power modification, are required.
Japanese Patent Laid-Open Publication No. 2001-160484 addresses the above-mentioned problems as in FIGS. 10 to 12.
In Japanese Patent Laid-Open Publication No. 2001-160484, a frequency of a resonant current passing through heating coil 18 and resonant capacitor 19 is set to be at least twice as high as that of driving signals fed to transistors 15, 17, in response to the signal from resonant current detector 30 for detecting a current passing through heating coil 18, thereby allowing for the heating of the aluminum pot by raising a frequency of the current supplied to heating coil 18, while suppressing the switching loss of the transistors 15, 17.
In an output control method for a low output power mode as shown in FIG. 11A, transistor 15 is turned off at a first instant when sign of collector current Ic1 thereof varies from positive value to zero and transistor 17 is turned off at a third instant when the sign of collector current Ic2 thereof varies from positive value to zero. Also, in a high output power mode as shown in FIG. 11B, transistor 15 is turned off at a second instant when the sign of collector current Ic1 thereof varies from positive value to zero and transistor 17 is also turned off at a second instant when the sign of collector current Ic2 thereof varies from positive value to zero.
Alternatively, in the low output power mode as shown in FIG. 12A, transistor 15 is turned off when time t1, which is shorter than a half period of the resonant current, elapses after transistor 15 is turned on and transistor 17 is turned off at a third instant when collector current Ic2 thereof decreases to zero from positive value. However, in the high output power mode as shown in FIG. 12B, transistor 15 is turned off at an instant when collector current Ic1 thereof drops to zero from positive value for the first time (turn-on time of transistor 15 corresponding to one half period of the resonant current) and transistor 17 is turned off at a third instant when the sign of collector current Ic2 thereof varies from positive value to zero.
The prior art induction heating apparatus of Japanese Patent Laid-Open Publication No. 2001-160484, however, suffers from certain drawbacks as follows. That is, a continuous output control cannot be achieved by the control method in FIGS. 11A, 11B, and a fine output control cannot be achieved by the control method in FIGS. 12A, 12B, because the variation of turn-on time produces too much variation of output power. Furthermore, because the envelope of current passing through heating coil 18 is not smoothed by the control methods of FIGS. 11A, 11B and FIGS. 12A, 12B, there occurs a pot vibration noise having a frequency of twice that of the commercial input power.
Japanese Patent Laid-Open Publication No. 1989-246783 addresses the problem of pot vibration noise generation, in which the output power is controlled by decreasing an input power fed to the inverter. However, even if this scheme is combined with the method disclosed in Japanese Patent Laid-Open Publication No. 2001-160484, suitable output control cannot be achieved because the resonant current is attenuated and thus cannot be maintained.
It is, therefore, an object of the present invention to provide an induction heating apparatus capable of heating an aluminum pot with a sufficiently large output power, in which the output power can be continuously adjusted with a fine controllability, while suppressing the generation of the pot vibration noise and switching loss in switching devices.
In accordance with the present invention, in case a load with a high conductivity and a low permeability is heated by a magnetic field generated by the heating coil, the resonant current passing through a switching device or a inverse-parallel diode (function as a reverse conducting device) resonates with a shorter period than a driving time of the switching device and further the DC voltage is boosted and smoothed by a boosting and smoothing circuit, and then provided for the inverter in order to maintain an amplitude of the resonant current to be higher than a certain value during the driving time, so that a switching loss of the switching device can be suppressed by lowering a driving frequency thereof, and at the same time the resonant current with higher frequency than the driving frequency thereof can be provided for the heating coil. Therefore, a load with a high conductivity and a low permeability, e.g., aluminum etc. can be heated with high output power.
Moreover, since the boosting and smoothing circuit for boosting and smoothing the input DC voltage fed to the inverter is provided to restrain the peak-to-peak value of the resonant current from attenuating to zero during the driving times of the switching device, in case of heating the load of high conductivity and low permeability, the output power can be stably controlled by varying the driving time of the switching device to be greater than one period of the resonant current and/or the burden (turn-on loss) of the switching device can be reduced.
In accordance with a first aspect of the present invention, there is provided an induction heating apparatus including:
an inverter having a switching device, a inverse-parallel diode (function as a reverse conducting device) connected to the switching device in parallel, a heating coil and a resonant capacitor, wherein the inverter generates a resonant current passing through the heating coil by turning on the switching devices;
a boosting and smoothing circuit; and
a control circuit for controlling a turn-on time of the switching device,
wherein in case a load with a high conductivity and a low permeability is heated by a magnetic field generated by the heating coil, the resonant current passing through the switching device or the inverse-parallel diode resonates with a shorter period than the turn-on time of the switching device and the DC voltage is boosted and smoothed by the boosting and smoothing circuit and then provided to the inverter in order to maintain an amplitude of the resonant current to be equal to or higher than a predetermined value during the turn-on time. Thus, a switching loss of the switching device can be suppressed by lowering a driving frequency thereof, and at the same time the resonant current with a higher frequency than the driving frequency can be provided for the heating coil. Therefore, a load with a high conductivity and a low permeability, e.g., aluminum etc. can be heated with a high output power.
Moreover, since the boosting and smoothing circuit for boosting and smoothing the input DC voltage fed to the inverter is provided to restrain the peak-to-peak value of the resonant current from attenuating to zero during the driving times of the switching device, in case of heating the load of high conductivity and low permeability, the output power can be stably controlled by varying the driving time of the switching device to be greater than one period of the resonant current and/or the burden (turn-on loss) of the switching device can be reduced.
In accordance with a second aspect of the present invention, there is provided an induction heating apparatus including:
an inverter including a resonant circuit having a first series connector containing a first switching device and a second switching device connected in series, a first inverse-parallel diode (function as a first reverse conducting device) connected to the first switching device in parallel, a second inverse-parallel diode (function as a second reverse conducting device) connected to the second switching device in parallel, and a second series connector, connected to the first and the second switching device in parallel, containing heating coil and a resonant capacitor, wherein the inverter resonates by turning on the first and the second switching devices;
a boosting and smoothing circuit; and
a control circuit for exclusively turning on the first and the second switching device,
wherein, in case a load with a high conductivity and a low permeability is heated by a magnetic field generated by the heating coil, the resonant current passing through the first switching device or the first inverse-parallel diode resonates with a shorter period than a turn-on time of the first switching device and the DC voltage is boosted and smoothed by the boosting and smoothing circuit, and then provided to the inverter in order to maintain an amplitude of the resonant current to be equal to or higher than a predetermined value during the turn-on time. And a burden of the switching devices can be reduced because two switching devices are used instead of only one, and at the same time, a fine and accurate output power control can be made according to the load by varying a ratio of driving times and/or a driving frequency of the switching devices.
Moreover, since the boosting and smoothing circuit for boosting and smoothing the input DC voltage fed to the inverter is provided to restrain the peak-to-peak value of the resonant current from attenuating to zero during the driving times of the switching devices, in case of heating the load of high conductivity and low permeability, the output power can be stably controlled by varying the driving times of the switching devices to be greater than one period of the resonant current and/or the burden (turn-on loss) of the switching devices can be reduced.
In accordance with a third aspect of the present invention, in particular, a boosting level of the DC voltage is determined by a turn-on time of at least one switching device included in the inverter. That is, by adjusting both the driving time and the boosting level, suitable output power control is made.
In accordance with a fourth aspect of the present invention, in particular, the boosting and smoothing circuit includes:
a smoothing capacitor connected in parallel to the first series connector including the first and the second switching device; and a choke coil connected to the second switching device in series,
wherein an energy is accumulated in the choke coil when the second switching device is turned on, and then the energy is transferred to the smoothing capacitor via the first inverse-parallel diode by turning off the second switching device. Thus, envelope of a pulsating DC voltage fed to the choke coil is smoothed and boosted, meanwhile the energy is accumulated at the second smoothing capacitor. And this smoothed DC voltage serving as a power source can be supplied to the resonant circuit including the first and the second switching device. Therefore, the induction heating apparatus described in the second aspect of the present invention can be embodied with simple circuit structure safely.
In accordance with a fifth aspect of the present invention, in particular, in case of heating the load with the high conductivity and the low permeability by the magnetic field generated by the heating coil, the resonant current passing through the second switching device or the second inverse-parallel diode resonates with a shorter period than a turn-on time of the second switching device. Therefore, the frequency of the resonant current can be increased easily with having equal distribution of burden between the first and the second switching device, so that the driving time (or turn-on time) of the second switching device becomes longer than the period of the resonant current. Thus, the amount of energy accumulated at the choke coil becomes larger and the boosting level can be increased, so that the operation described in the second aspect of the present invention, i.e., the operation, a peak-to-peak value of the resonant current passing through the first switching device can be controlled not to come down to zero during the driving time of the first switching device, can be embodied easily.
In accordance with a sixth aspect of the present invention, in particular, high frequency components on accumulating the energy at the choke coil can be prevented from leaking into the power source by having an additional smoothing capacitor for giving an energy to the choke coil when the second switching device is turned on.
In accordance with a seventh aspect of the present invention, in particular, in the maximum output power mode, the control circuit outputs either a turn-off signal of the first switching device while the resonant current is passing therethrough after a start of the second period of the resonant current ensuing after turning on the first switching device, or a turn-off signal of the second switching device while the resonant current is passing therethrough after a start of the second period of the resonant current appearing after turning on the second switching device. Therefore, the turn-on loss of the second and the first switching device can be reduced in the maximum output power mode.
In accordance with a eighth aspect of the present invention, the control circuit outputs, in the maximum output power mode, either a turn-off signal of the first switching device during a period when the resonant current decreases from its peak value to zero after a start of the second period of the resonant current appearing after turning on the first switching device, or a turn-off signal of the second switching device during a period when the resonant current decreases from its peak value to zero after a start of the second period of the resonant current appearing after turning on the second switching device. Therefore, the first and the second switching device can be turned off when the resonant current is passing therethrough. Moreover, the first and the second switching device can be turned on when the resonant current is passing through the first and the second inverse-parallel diode in a forward direction, respectively.
In accordance with a ninth aspect of the present invention where a load of high conductivity and low permeability is heated by a magnetic field generated by the heating coil, the first resonant current passing through the first switching device and the first inverse-parallel diode or the second resonant current passing through the second switching device and the second inverse-parallel diode resonates with a period being approximately ⅔ of the driving time of the first or the second switching device, so that the switching devices are turned off when the resonant current reaches at second peak. Therefore, the amount of resonant current at the time of turning off either one of the switching devices becomes larger than that of the current at the time of turning off either one of the switching devices at the third peak of the resonant current.
Thus, after turning off the second switching device, a stable commutation is carried out easily for the current to pass through the first inverse-parallel diode in its forward direction, and the occurrence of the turn-on mode of the first switching device is prevented, resulting in a reduction of a switching loss and a high-frequency noise. Similarly, such also occurs in the second switching device and the second inverse-parallel diode, after turning off the first switching device. In case of a fourth or a fifth aspect of the present invention, which will be described hereinafter, the driving time of the second switching device becomes longer than that of the resonant current, so that the amount of energy accumulated at a choke coil increases. Thus, the boosting level also increases, so that above-mentioned operations can be carried out more efficiently.
In accordance with the tenth aspect of the present invention where the load of high conductivity and low permeability is heated by a magnetic field generated by the heating coil, the ratio of driving times of the first and the second switching device is set at 1 approximately, and the resonant current passing through the first switching device or the first inverse-parallel diode resonates with a period being approximately ⅔ of the driving time of the first switching device. Therefore, the first and the second switching device are turned on when the resonant current is passing through the first and the second inverse-parallel diode in their forward direction and at the same time, the first and the second switching device are turned off when the resonant current is passing through the first and the second switching device in their forward direction.
Moreover, since the resonant current resonates with the period of approximately ⅔ of the driving time of the first and the second switching device, switching devices can be turned off around the second peak of the resonant current. Therefore, switching devices can be turned off when the resonant current is attenuated by a small amount. Thus, a commutation is carried out stably, for the resonant current to pass through the second and the first inverse-parallel diode in their forward direction after turning off the first and the second switching device, so that the turn-on mode of the switching devices can be restrained from occurring and a switching loss and a high-frequency noise thereof can be avoided. Further, the resonant current with a high frequency of 3 times as high as the driving frequency of the switching devices can be provided for heating coil.
In accordance with the eleventh aspect of the present invention, in starting a heating operation, an output power is increased by varying the ratio of driving times of the first and the second switching device and then by varying the driving frequency, thus resulting in easy detection of the load. That is to say, an output power transmitted to either a load of high conductivity and low permeability like aluminum etc., or an iron based load can be varied steadily in the low output power mode by varying the ratio of driving times, and thus the load can be detected accurately in the low output power mode.
Moreover, after reaching a predetermined ratio of driving times, driving time, or output power, the ratio of driving times is set at a constant value in order to drive and turn off the switching devices within a specific range of phase in the case of the load of high conductivity and low permeability. While maintaining the ratio of driving times at constant value, a turn-off phase and the driving frequency are changed, so that an output power can be adjusted without significantly increasing the loss of switching devices.
In accordance with a twelfth aspect of the present invention, upon initiating the heating operation, the driving time of the first switching device is set to be shorter than the resonant period of the resonant current and then an output power is increased by changing the ratio of driving times of the first and the second switching device until a certain driving time or a certain ratio of driving times is reached. During that time, it is accurately and safely detected whether or not the load is of high conductivity and low permeability. In case the load is detected to be of high conductivity and low permeability, the driving time of first switching device is dispersedly increased to lower the output power, and then the output power is stably increased from the low level to a desired level by steadily increasing the length of the driving time.
In accordance with a thirteenth aspect of the present invention, in case of heating iron-based load or load of a non-magnetic by the magnetic field generated by the heating coil, the resonant current resonates with a longer period than the driving time of the first and the second switching device. And in case the load of iron-based material or non-magnetic stainless steel is heated with a maximum output power, a resonance compensation capacitor is connected to the resonant capacitor in parallel, resulting in larger capacitance than that of the case when a load is of high conductivity and low permeability, in order to turn off the first and the second switching device at the time when a current passes through the first and the second switching device in a forward direction. Thus in case of the load of iron-based material or non-magnetic stainless steel, the resonant period becomes longer and at the same time the resonant current is increased. Further, since DC voltage Vdc is boosted by the choke coil, an amplitude of the resonant current becomes larger. Therefore, the maximum output power can be made to be larger than that of the prior art, in case the turn-on switching loss is suppressed by setting up the maximum output power within the range which enables the switching devices to be turned off at the time a current is passing through the switching devices in their forward direction.
In the prior art induction cooking apparatus, the selective heating of an aluminum based pot and an iron based pot using a same inverter was made by changing the number of turns of the heating coil in order to change the intensity of magnetic field (ampere-turn) transmitted to the load. In accordance with the present invention, however, the effect of converting the number of turns is achieved by the boosting operation of the second switching device and the choke coil, and the resonant capacitance is adjusted through the use of the resonance compensation capacitor, so that load of wide range of materials can be heated by using the same heating coil.
In accordance with a fourteenth aspect of the present invention, the operation of the embodiment of the present invention is started with no connection of the resonance compensation capacitor to the resonant capacitor, i.e., with lower capacity, and an output is increased by degrees, meanwhile load is detected to be whether it is of iron or of high conductivity and low permeability. If load is found to be iron, the operation thereof is stopped and the resonance compensation capacitor is connected to the resonant capacitor in parallel by turning on a relay, i.e., higher capacity and the driving frequency is set to be low frequency again.
However, if load is detected to be of high conductivity and low permeability, the output is increased until certain ratio of driving times or certain output power is reached, and then the ratio of driving times is fixed but the driving frequency of switching device is varied, to thereby reach a suitable output power. Therefore, according to the result of discrimination between a load of high conductivity and low permeability and a load of iron based, with low output power, suitable resonant capacitor and suitable driving method are chosen, thereby achieving a suitable output power.